The present invention is broadly concerned with low shear extrusion and drying apparatus and methods of extrusion of various products such as starches and proteinaceous products. In one aspect of the invention, methods of forming granular cold water soluble starch products (which can also be referred to as cold water swelling products) while producing little or no liquid effluent are provided. In this way, large quantities of the starch products can be produced in shorter periods of time compared to prior art processes. The invention is also directed towards methods of forming pre-gelled starch products and of processing starch-bearing materials which normally yield sticky, unworkable products using little or no surfactant, amylose-complexing agents, or lipids. In practice, an extruder is coupled with an elongated, tubular die assembly and processing conditions are selected so as to substantially cook the starch-bearing materials. This involves product residence times within the downstream die assembly which are greatly in excess of extruder residence times. In one embodiment, as the cooked extrudate emerges from the die assembly, it is passed through a dehydration assembly which substantially removes the liquids from the extrudate to yield a dried, solid cold water soluble starch product.
Starch-bearing materials such as prime starches and farinaceous grains have long been extrusion processed. For example, food thickeners can be made by extrusion techniques, although they generally do not have thickening powers equal to drum dried starches. Likewise, grains can be extrusion processed to give a number of useful end products. Generally speaking, extrusion of starch-bearing products, if not carefully controlled, can lead to wholly unworkable, sticky extrudates which cannot be properly sized, shaped or handled. It is known that extrusion processing and the shear conditions inherent therein tend to degrade the starch which, in the presence of moisture, leads to stickiness or functionality problems. It has been determined that shear conditions are a major contributing factor to the problems with prior extrusion processing of starches. Excess shear can lead to starch granule damage, vitamin losses, protein complexing or degradation, and reduction in molecular weight of biopolymers.
In order to overcome this issue, it has heretofore been thought necessary to add appreciable quantities of surfactants such as monoglycerides or lipids or derivatives thereof to the extrusion mixture. Although surfactant use can control the stickiness problem, the added surfactants are relatively expensive and add significant cost to the final product.
In the past, instant, non-granular starches have traditionally been formed using conventional drum-drying techniques. Generally, these starches have been chemically modified in order to achieve the desired gelling properties. U.S. Pat. No. 4,465,702 to Eastman et al. discloses a cold water soluble granular starch derived from chemically unmodified, ungelatinized corn starch. This starch is prepared by forming a slurry of corn starch in a liquid processing medium comprising water and alcohol. The slurry is heated to a temperature of at least 300xc2x0 F. at or above autogenic pressure, after which the starch material is separated from the processing medium. However, the Eastman et al. process is a batch process, thus preventing large quantities of cold water soluble starch from being rapidly produced. Furthermore, the Eastman et al. process produces rather large quantities of liquid effluent.
In like manner, extrusion processing of proteinaceous products and nutraceuticals have presented problems such as protein complexation or degradation of proteins and vitamins or other nutraceuticals. Here again, it is believed that high shear conditions found in many extruders are the chief reasons why extrusion processing can create problems of these types.
There is accordingly a need in the art for imnproved, low shear extrusion apparatus and corresponding methods which can be used to process a wide variety of products while minimizing shear-related problems. For example, there is a need for a process for continually producing cold water soluble or swellable starches with little or no liquid effluent and with a minimal amount of shear.
The present invention provides improved extrusion apparatus and methods which yield improved end products. The invention is suitable for use in the processing of protein-bearing substances (e.g., vegetable, dairy or meat protein products) as well as processing of starch-bearing materials. In the latter case, the methods here of product workable, starch-bearing normally sticky extrudates with little or no surfactant, lipids or derivatives thereof. Indeed, in preferred forms of this embodiment, the starch-bearing materials are essentially free of any such added ingredients. The invention further provides improved methods for forming pre-gelled starch products. In another embodiment, the invention provides extrusion and drying apparatus and methods for forming cold water soluble/swelling/gelling starch products. As used herein, soluble, swelling, and/or gelling starch refers to a starch which will form, in the presence of sufficient water and without additional heating or cooking, a coherent starch mass which is sliceable and has the appearance of a fully cooked starch gel. In the case of pre-gelled starches, while they will form a coherent starch mass in the presence of water without heating or cooking, some will not be sliceable nor have the appearance of a fully cooked starch gel.
Broadly speaking, extrusion cooking apparatus in accordance with all embodiments of the invention are in the form of an elongated extruder having a tubular barrel with an outlet end and at least one (and preferably a pair of) elongated, axially rotatable, helically flighted screw(s) within the barrel for moving material therethrough. A tubular die assembly is operatively coupled to the barrel adjacent the outlet end thereof and includes an elongated, tubular body with an apertured die secured to the end of the tubular body remote from the extruder barrel.
Preferably, the extrusion apparatus is designed so that the residence or retention time of the product within the die assembly is greater than the residence time within the extruder. Thus, the product is subjected to reduced shear during processing so as to minimize shear-related problems, e.g., the tendency of starch-bearing materials to become sticky and unworkable. At the same time, the extrusion conditions should be sufficient to adequately cook the product, which in the case of starch-bearing materials generally means that the materials have essentially completely gelatinized starch fractions (preferably at least about 80% gelatinized, and more preferably at least about 100% gelatinized). The volumetric ratio of the die assembly to the extruder free volume should be at least about 2, preferably from about 3-20, and more preferably from about 3-6.
When forming cold water soluble starch products, a dehydrating assembly is preferably positioned adjacent the die for receiving and at least partially dehydrating an extrudate. Preferably, the dehydrating assembly comprises an agitator and an upright dehydrating tower operatively coupled to receive material from the agitator.
In accordance with the apparatus and methods of the invention, cold water soluble and pre-gelled starch products can be continuously produced. These products can be formed utilizing starches selected from the group consisting of grain, root and tuber starches, rice, wheat, oats, barley, corn, rye, and mixtures thereof; in addition, whole grain products can also be produced from the foregoing types of starch-bearing grains.
Cold water soluble starch products in accordance with the invention have a water solubility of at least about 10%, and preferably from about 60-70%. As used herein, the cold water solubility of a starch-containing sample is determined by mixing 1 gram of the sample with 100 ml of distilled water in a Waring Blender for 15 seconds at low speed and 2 minutes at high speed. The suspension is then centrifuged at 3100 rpm for 15 minutes and dried at 110xc2x0 C. for 4 hours thus evaporating the supernatant liquid. The residue is then weighed and compared to the starting weight of the sample to determine the percent of the sample that was solubilized in the water.